Go around the the mountain where the bomb is hanging and follow the path up and around the back of the observatory. In a crevice in the side is a ladder to climb to the top of the building ( you probably saw this while trying to find a way in ). When your on the top of the observatory, keep looking inside the windows to the INSIDE of the observatory ( where you are trying to get into ) until you see a red eye painted on the wall with blood from it. To the right of that eye ( the direction of the blood ) is a small switch. Shoot it and the electric fence will turn off. You can now drop down and go in. You will be attacked by some mobs as you go around and at the top of the stairs there is a red Chest to loot.